Add self-contained CodeReference types file, compat checks, and docs download endpoint

[tbuckley]

Motivation

• Provide a single, copy/download-ready TypeScript file containing all CodeReference-related types so consumers can paste or import definitions without depending on internal $lib modules.
• Ensure the downloadable file is fully self-contained (no imports) so end users can use it outside the repo.
• Keep documentation types and runtime types aligned by adding a compile-time compatibility check.

Description

• Add src/lib/documentation/codeReferenceTypes.ts, a self-contained, interface-focused TypeScript definitions file that declares FileReference, CodeReference, prompt/provider shapes, Fs* storage types, JavascriptAssertionArgs, and a Generator interface.
• Add src/lib/documentation/codeReferenceTypes.compat.ts, a type-level compatibility file that compares the downloaded doc types against the repo runtime types to catch regressions.
• Export and align missing runtime types used by the doc bundle: export Generator in src/lib/storage/runGenerators.ts, add FsProvider in src/lib/storage/types.ts, and export JavascriptAssertionArgs from src/lib/assertions/javascript.ts.
• Add a docs page and a downloadable endpoint at src/routes/documentation/code-reference-types/+page.svelte and src/routes/documentation/code-reference-types/types/+server.ts to display and serve the codeReferenceTypes.ts file, and link it from the main docs (src/routes/documentation/+page.md).

Testing

• CI=1 npm run test was executed and all tests passed (integration + unit).
• npm run check (svelte-check / type checking) completed with no errors.
• npm run lint (Prettier / ESLint / tsc --noEmit) completed successfully after formatting and small type adjustments.

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Codex Task

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